Method, system and apparatus for accurate and stable LC-based reference oscillators

ABSTRACT

A substantially temperature-independent LC-based oscillator is achieved using an LC tank that generates a tank oscillation at a phase substantially equal to a temperature null phase. The temperature null phase is a phase of the LC tank at which variations in frequency of an output oscillation of the LC-based oscillator with temperature changes are minimized. The LC-based oscillator further includes frequency stabilizer circuitry coupled to the LC tank to cause the LC tank to oscillate at the phase substantially equal to the temperature null phase.

CROSS REFERENCE TO RELATED PATENTS

This U.S. application for patent claims the benefit of the filing dateof U.S. Provisional Patent Application entitled, Method, System andApparatus for Accurate and Stable LC-Based Reference Oscillators, havingSer. No. 61/189,809, filed on Aug. 23, 2008, which is incorporatedherein by reference for all purposes.

BACKGROUND OF THE INVENTION

1. Technical Field of the Invention

The present invention relates in general to oscillators, and inparticular, to inductor-capacitor (LC) tank based oscillators.

2. Description of Related Art

Electronic clock generation classically relies on a reference oscillatorbased on an external crystal that is optionally multiplied and/ordivided to generate the required clock. The key specifications of aclock, other than its target frequency, are frequency accuracy andstability. Frequency accuracy is the ability to maintain the targetfrequency across supply and temperature and is usually represented asdrift from the target frequency in percent or ppm. Short term frequencystability is measured using period jitter which is dependent on thephase noise of the oscillator at large frequency offsets. Long termstability, on the other hand, is impacted by the close-in phase noise ofthe oscillator. An oscillator using a high-Q element typically has a lowphase noise profile, and thus good frequency stability, and is lesssensitive to variations in oscillator amplifier gain that is dependenton supply and temperature.

For example, crystal oscillators (XO) are high-Q oscillators thatprovide excellent frequency stability and frequency accuracy acrosssupply and temperature stemming from the very high quality factor (Q) ofthe crystal. However, not all resonators, including crystals, havesatisfactory performance across temperature, thus the need for extracircuitry and techniques to decrease and/or compensate for this shift infrequency due to temperature. A temperature compensated crystaloscillator (TCXO) typically incorporates extra devices that havetemperature dependence to negate the temperature dependence of thecrystal. The overall outcome is an oscillation frequency with lowtemperature dependence.

However, the ever increasing complexity of electronic systems due torequirements of supporting multiple standards, increased functionality,higher data rates and memory in a smaller size and at a lower cost ispushing designers to increase the integration level through thedevelopment of Systems on Chip (SoC) in deep submicron Complimentary MOS(CMOS) technologies to benefit from the increased gate density.Reference clocks incorporating crystal oscillators have not managed toscale or integrate due to the bulky nature of crystals, thus limitingthe size and cost reduction possible for electronic systems.

Recent efforts in using high-Q MEMS resonators and Film Bulk AcousticResonators (FBARs) have illustrated possibilities of integrating ahigh-Q element and Application Specific Integrated Circuits (ASIC) inthe same package. However, packaging induced stress impactingperformance still remains as a challenging obstacle, since the high-Qelement may require special packages and/or calibration that are notpractical for SoC's. The stress may change the temperature behavior ofthe resonator, possibly resulting in large frequency shifts andaccelerated aging. Therefore, special assembly and packaging techniquesare typically required to mitigate such effects, which increase the costof producing such clocks. Similar problems may be encountered by anyresonator that is dependent on the mechanical properties of theresonator material, which require careful design and manufacturingprocedures and processes.

Design requirements for applications such as USB and SATA, which do notrequire superior frequency accuracy and stability can be satisfied usingoscillators with relatively low-Q elements available in a CMOS processwhich can have adequate phase noise profiles generating good jitterperformance. Current trials include the use of ring oscillators,relaxation oscillators and LC oscillators. However, the reportedfrequency accuracy of these implementations suffers from large driftacross supply and temperature, making them ineffective for applicationsrequiring precise accuracy and stability. A mitigation to reduce thedrift across temperature requires trimming across temperature which isneither cost effective nor practical for SoC's.

Therefore, the availability of an integrated solution relying onexisting optimized process steps in a CMOS technology satisfyingfrequency stability and jitter requirements is of great value.

SUMMARY OF THE INVENTION

Embodiments of the present invention provide a substantiallytemperature-independent LC-based oscillator. The oscillator includes anLC oscillator tank that generates a tank oscillation at a phasesubstantially equal to a temperature null phase. The temperature nullphase is a phase of the LC oscillator tank at which variations infrequency of an output oscillation of the oscillator with temperaturechanges are minimized. The oscillator further includes frequencystabilizer circuitry coupled to the LC oscillator tank to cause the LCoscillator tank to operate at the temperature null phase.

For example, in one embodiment, the temperature null phase issubstantially equal to zero and the frequency stabilizer circuitryincludes a series capacitive resistor within the LC oscillator tankhaving a value selected to enable a quality factor of the inductor (L)and capacitor (C) to be substantially equal across a temperature rangeof interest.

In another embodiment, the temperature null phase is non-zero, and thefrequency stabilizer circuitry includes phase shift circuitry thatoperates to produce a phase substantially equal to a negative of thetemperature null phase based on a phase control signal.

In yet another embodiment, the oscillator includes a first amplifieroperable to inject a first current into the LC oscillator tank, and thefrequency stabilizer circuitry includes a second amplifier coupled tothe LC oscillator tank and operable to inject a second current into theLC oscillator tank. A phase shift between the first current and thesecond current produces a non-zero value of the phase in the LCoscillator tank based on a ratio between the first current and thesecond current. This ratio is chosen such that the oscillator operatesat the temperature null phase.

In still another embodiment, two oscillators are coupled to one anotherin quadrature to enable each oscillator to operate at its respectivetemperature null phase. Each oscillator includes two amplifiers coupledto inject a combined current into the respective LC oscillator tank thatcauses the LC oscillator tank to oscillate at a non-zero phase which isset to be substantially equal to the temperature null phase. The phaseis set based on the ratio between the currents injected by eachamplifier. In a further embodiment, automatic amplitude controlcircuitry measures the amplitude of the respective outputs of theoscillators and generates a control signal to control the oscillationamplitude such that the amplifiers are operating in their linear region,thus decreasing any harmonics which may have an adverse affect on theoscillator temperature dependence.

In another embodiment, the frequency stabilizer circuitry furtherincludes a phase locked loop (PLL) or delay locked loop (DLL) having aphase detector coupled to receive the output oscillation and a feedbacksignal and operable to generate a phase error output signal indicativeof a difference in phase between the output oscillation and the feedbacksignal, in which the phase error output signal is equal to zero when theoutput oscillation and the feedback signal are shifted by a constantphase. A Voltage Controlled Oscillator (VCO) or Voltage Controlled DelayLine (VCDL) receives the filtered phase error output and produce thefeedback signal. A programmable coupling circuit coupled to VCO/VCDLprovides a fraction of the feedback signal to the oscillator to causethe LC oscillator tank to oscillate at a non-zero phase. The couplingfactor is chosen such that the oscillator tank has a phase substantiallyequal to the temperature null phase.

In another embodiment, the frequency stabilizer circuitry includes aheating element operable to apply a temperature excitation signalsuperimposed on an operating temperature of the oscillator, ademodulator coupled to the oscillator to receive the output oscillationand operable to demodulate the output oscillation to produce a frequencydeviation signal indicative of a frequency deviation produced by theoscillator in response to the temperature excitation signal, a frequencyerror generator coupled to receive the frequency deviation signal andthe temperature excitation signal and operable to produce an errorsignal proportional to the sensitivity of the oscillator frequency totemperature and a comparator coupled to receive the error signal and areference signal and operable to produce a control signal to control theoscillator to allow the LC oscillator tank to operate at the temperaturenull phase. At steady state, the error signal has an average value equalto the reference signal.

Embodiments of the present invention further provide a method foroperating an LC-based oscillator. The method includes determining atemperature null phase of an LC oscillator tank of the LC-basedoscillator, in which the temperature null phase is a phase at whichvariations of frequency of an output oscillation of the LC-basedoscillator with temperature changes are minimal. The method furtherincludes providing frequency stabilizer circuitry to cause the LCoscillator tank to operate at a phase substantially equal to thetemperature null phase and operating the LC-based oscillator with the LCoscillator tank oscillating at the temperature null phase.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete understanding of the present invention may be obtainedby reference to the following detailed description when taken inconjunction with the accompanying drawings wherein:

FIG. 1 is a circuit diagram illustrating an exemplary LC oscillator tankin accordance with embodiments of the present invention;

FIG. 2 illustrates phase plots of an exemplary LC oscillator tank inaccordance with embodiments of the present invention;

FIG. 3 illustrates the oscillation frequency across temperature of an LCoscillator tank operating at zero phase;

FIG. 4 illustrates a temperature null phase of an exemplary LCoscillator tank in accordance with embodiments of the present invention;

FIG. 5 illustrates frequency deviation across temperature whileoperating at the temperature null phase in accordance with embodimentsof the present invention;

FIG. 6 is a block diagram illustrating an exemplary oscillator having anLC oscillator tank operating at the temperature null phase in accordancewith embodiments of the present invention;

FIG. 7 is a flow diagram illustrating an exemplary method for operatingan LC-based oscillator in accordance with embodiments of the presentinvention;

FIG. 8 is a circuit diagram illustrating an exemplary LC oscillator tankoperating at the temperature null phase in accordance with embodimentsof the present invention;

FIG. 9 is a flow diagram illustrating a method for operating an LC-basedoscillator at a non-zero phase in accordance with embodiments of thepresent invention;

FIG. 10 is a diagram illustrating an exemplary oscillator designed tooperate the LC oscillator tank at the temperature null phase inaccordance with embodiments of the present invention;

FIG. 11 is a diagram illustrating another exemplary oscillator designedto operate the LC oscillator tank at the temperature null phase inaccordance with embodiments of the present invention;

FIG. 12 is a diagram illustrating an exemplary oscillator designed tooperate with minimal temperature dependence in accordance withembodiments of the present invention;

FIG. 13 is a diagram illustrating an exemplary oscillator with anAutomatic Amplitude Control (AAC) block accordance with embodiments ofthe present invention;

FIG. 14 is a diagram illustrating still another exemplary oscillatordesigned to operate the LC oscillator tank at the temperature null phasein accordance with embodiments of the present invention;

FIG. 15A is a diagram illustrating a feedback aided oscillator designedto operate the LC oscillator tank at the temperature null phase inaccordance with embodiments of the present invention;

FIG. 15B is a diagram illustrating an exemplary design of a frequencyerror generator for use within the feedback aided oscillator of FIG. 15Ain accordance with embodiments of the present invention;

FIG. 15C illustrates the stabilization point of the feedback aidedoscillator of FIG. 15A in accordance with embodiments of the presentinvention;

FIG. 15D is a circuit diagram illustrating an exemplary heater circuitfor use within the feedback aided oscillator of FIG. 15A in accordancewith embodiments of the present invention;

FIG. 15E illustrates an exemplary temperature signal emitted from theheater circuit shown in FIG. 15D in accordance with embodiments of thepresent invention;

FIG. 16A is a circuit diagram illustrating an exemplary differential LCoscillator tank for producing an extended temperature null phase rangein accordance with embodiments of the present invention;

FIG. 16B illustrates an exemplary extended temperature null phase rangeproduced by the differential LC oscillator tank of FIG. 16A inaccordance with embodiments of the present invention;

FIGS. 17A and 17B illustrate exemplary temperature trimming operationsin accordance with embodiments of the present invention;

FIGS. 18A and 18B illustrate an exemplary oscillator output spectrumwith and without temperature modulation; and

FIG. 19 is a flow diagram illustrating a method for determining thefrequency sensitivity of an oscillator to temperature in accordance withembodiments of the present invention.

DETAILED DESCRIPTION OF THE DRAWINGS

Referring now to FIG. 1, an LC oscillator tank circuit 10 used inbuilding oscillators is composed of a source of inductance (inductiveelement) L and capacitance (capacitive element) C. The inductive elementL and capacitive element C in the LC oscillator tank circuit 10 can becomposed of various types of resonators and passive elements, such asbut not limited to, on-chip integrated inductors, bond-wires,Metal-Insulator-Metal (MiM) Capacitors, Metal Finger Capacitors, MetalOxide Semiconductor (MOS) capacitors, ceramic resonators, MicroElectro-Mechanical Systems (MEMS) tuning fork resonators, MEMSwine-glass resonators, MEMS-based resonators, Surface Acoustic Wave(SAW) and Bulk Acoustic Wave (BAW) devices.

The implementation of an ideal pure inductor or capacitor is usually notpossible due to the physical limitations of having a finite qualityfactor Q. Integrated inductors in CMOS technology to date have low Qfactors when compared to MEMS resonators and crystals. Of the sources oflosses in an inductor are the inductor metal ohmic losses r_(L) andsubstrate resistive losses r_(SUB). Both of these losses are usuallytemperature dependent, and therefore, the overall impedance and Q of theinductor is temperature dependent.

The integrated capacitive part of the tank also suffers from a finitetemperature dependent Q as well as temperature dependence of thecapacitance value. As a result, the physical implementation of anintegrated LC-tank will dictate a strong temperature dependence of theimpedance and Q factor of the tank, which may result in a temperaturedependent tank resonance frequency.

Thus, an oscillator built using an LC oscillator tank 10 typicallyincludes an amplifier responsible for overcoming the tank losses. Forthe oscillator to have sustained oscillations, the Barkhausen criterionrequires an open loop gain greater than 1 and phase equal to zero.Assuming that the amplifier contributes a zero phase, then foroscillation to occur, the LC oscillator tank impedance Z_(TANK) musthave a zero phase. The phase condition is used to derive the oscillationfrequency ω_(osc) as follows:

$\begin{matrix}{{Z_{Tank} = \frac{r_{L} + {{j\omega}\; L}}{1 + {{j\omega}\; r_{L}C} - {\omega^{2}{LC}}}},} & \left( {{Equation}\mspace{14mu} 1} \right) \\{{\phi_{Tank} = {\tan^{- 1}\frac{\omega \; L}{r_{L}}\tan^{- 1}\frac{\omega \; r_{L}C}{1 - {\omega^{2}{LC}}}}},} & \left( {{Equation}\mspace{14mu} 2} \right)\end{matrix}$

with an oscillation condition of φ_(Tank)=0 such that:

$\begin{matrix}{{\omega_{osc} = {\omega_{o} \cdot \sqrt{1 - \frac{r_{L}^{2}C}{L}}}},{{{where}\mspace{14mu} \omega_{o}} = {\frac{1}{\sqrt{LC}}.}}} & \left( {{Equation}\mspace{14mu} 3} \right)\end{matrix}$

From the above equations 1-3, it can be seen that the oscillationfrequency is temperature dependent if r_(L) is temperature dependent. Alinear variation of r_(L) with temperature results in an almost linearvariation of the oscillation frequency. In addition, any temperaturevariation in C would strongly contribute to the temperature dependence.

This is graphically shown in FIG. 2, where φ_(Tank) plotted fordifferent temperatures for a tank composed of L, r_(L) and C with lineartemperature dependence of r_(L) defined as:

r _(L) =r _(o)·(1+α·(T−T _(o))),  (Equation 4)

where α is the temperature coefficient of r_(L).

It is to be noted that the oscillation frequency is determined using theintersection of φ_(Tank)=0 with the phase plots. The correspondingoscillation frequency across temperature is plotted in FIG. 3, showing astrong temperature dependence of 8000 ppm in a typical LC oscillatortank operating at zero phase.

Examining the phase plots again in FIG. 2, since the quality factor ofthe tank changes with temperature, the phase plots change withtemperature. In addition, at the oscillation frequency, there is ahigher Q at lower temperatures, and therefore, a larger negative slopeis seen at lower temperatures. Therefore, the varying phase plot slopeswith temperature result in the intersection of these plots.

When the intersections occur at the same phase, a temperatureinsensitive tank operating point is created, and the tank is said to beoperating at a temperature “null” with a phase φ_(Null). The idealtemperature null phase occurs when the phase plots across temperatureintersect at precisely the same phase. Oscillation with a phase acrossthe tank of and ideal φ_(Null) results in an oscillation frequency withzero deviation across temperature.

More realistic tanks exhibit a temperature null with small frequencydeviations across temperature. This is illustrated graphically in FIG.4, where the condition of oscillation is φ_(Tank)=φ_(Null) and thecorresponding oscillation frequency across temperature is plotted. Ascan be seen in FIG. 5, operating the oscillator at the temperature nullphase results in an oscillation frequency with a much lower temperaturedependence. For example, in FIG. 5, the frequency drift is only 290 ppm.Comparing this to the 8000 ppm frequency drift at zero phase in FIG. 3,oscillating at the temperature null phase produces a more stablefrequency.

A Global Temperature Null (GNull) can be defined as a phase operatingpoint φ_(GNull) that results in a minimum frequency deviation Δf acrossa temperature range ΔT with a very small or zero change in oscillationfrequency over temperature df_(osc)/dT at the center of the temperaturerange T_(o). A measure of the quality of the temperature null is theoscillation frequency deviation across temperature. A Figure of Merit(FOM) of the tank temperature null may be defined as:

$\begin{matrix}{{{F\; O\; M} = {\frac{\Delta \; f}{{f_{To} \cdot \Delta}\; T}\mspace{14mu} {{ppm}/{^\circ}}\mspace{14mu} {C.}}},} & \left( {{Equation}\mspace{14mu} 5} \right)\end{matrix}$

where f_(To) is the oscillation frequency at T_(o). The smaller thevalue of the FOM the better the null quality and the perfect null hasFOM=0.

A Local Temperature Null (LNull) can be defined as a phase operatingpoint φ_(LNull) with df_(osc)/dT=0. Alternatively, LNull can be definedat temperature T as the intersection of the phase plots of temperaturesT+δ and T−δ where δ is infinitesimally small.

The GNull oscillation frequency ω_(GNull) around temperature T_(o) maybe derived by finding the intersection of two phase curves attemperatures (T_(o)+ΔT) and (T_(o)−ΔT). For an LC oscillator tank with alinear temperature dependence of r_(L) the phase and frequency at theGNull are as follows:

$\begin{matrix}{{\omega_{GNull} = {\omega_{o} \cdot \sqrt{1 + {\frac{C}{L}{r_{o}^{2}\left( {1 - {\alpha^{2}\Delta \; T^{2}}} \right)}}}}}{{and}\mspace{14mu} {therefore}\text{:}}} & \left( {{Equation}\mspace{14mu} 6} \right) \\{\phi_{GNull} = {- {\tan^{- 1}\left( {2r_{o}C\; \omega_{GNull}} \right)}}} & \left( {{Equation}\mspace{14mu} 7} \right)\end{matrix}$

From the above equations 6 and 7, it can be seen that ω_(GNull)>ω_(o)and is a weak function of ΔT. Furthermore, φ_(LNull) is negative and isa very weak function of ΔT signifying that the LNull at T_(o) (ΔT=0) isthe center of the GNull with a temperature range of (2.ΔT) such that:

φ_(GNull)=φ_(LNull)|_(T=To)  (Equation 8)

Thus, it is possible to locate a GNull using an LNull point in thetemperature range of interest.

The FOM of a GNull generally improves as the Q of the tank increases andthe temperature coefficient of the inductor ohmic losses, α, decreases.However, it is possible to improve the FOM of a GNull by using atemperature dependent active and/or passive capacitance. For example,referring again to FIG. 1, capacitor C can be a temperature dependentcapacitor. The oscillation frequency is dependent on the impedance ofthe tank with the capacitive part being very effective such that asecond order or higher temperature dependence of the capacitor may actto decrease the overall frequency deviation across the temperature rangeof interest.

Referring now to FIG. 6, classically, LC oscillators 5 have beendesigned to operate with very close to zero phase across the LCoscillator tank 10, thus usually missing the temperature null. In orderto satisfy the Barkhausen criterion, as shown in FIG. 6, in accordancewith embodiments of the present invention, a frequency stabilizercircuitry 20 can be used to enable the LC oscillator 5 to oscillate atzero open loop phase, while the LC oscillator tank 10 oscillates at thetemperature null phase. For example, in one embodiment, the frequencystabilizer circuitry 20 can be included within the LC oscillator tank 10to move the temperature null phase to near zero. In other embodiments,the frequency stabilizer circuitry 20 can cause the LC oscillator tank10 to oscillate at a non-zero value of the temperature null phase, whileproviding an equal and opposite phase to the temperature null phase toenable the LC oscillator 5 to effectively oscillate at zero open loopphase, thus satisfying the Barkhausen criterion. The frequencystabilizer circuitry 20 can also include a controller that operates toset the phase of the LC oscillator tank to the temperature null phaseduring calibration/trimming operations and/or during real-timeoperations. Various embodiments of the LC oscillator tank 10 andfrequency stabilizer circuitry 20 will be described below in connectionwith FIGS. 8 and 10-15.

FIG. 7 is a flow diagram illustrating an exemplary method 100 foroperating a temperature-independent LC-based oscillator in accordancewith embodiments of the present invention. At block 110, the LCoscillator tank is provided and its temperature null phase is estimated.The temperature null phase is the phase at which variations of frequencywith temperature changes are minimal. At block 120, frequency stabilizercircuitry is provided to cause the LC oscillator tank to oscillate at aphase substantially equal to the temperature null phase such that theoscillator has a zero open loop phase, while at block 130, the LC-basedoscillator is operated with the LC oscillator tank oscillating at thephase substantially equal to the temperature null phase. For example, inone embodiment, the LC-based oscillator can be trimmed to select thephase that is substantially equal to the temperature null phase toproduce a substantially temperature-independent oscillation frequencyacross a specific temperature range.

Referring now to FIG. 8, in order to use classical oscillatorimplementations and benefit from the temperature null, the LC oscillatortank 10 can be designed to have a φ_(GNull)˜0. This can be achieved byhaving the quality factor of the inductor Q_(L) and the quality factorof the capacitor Q_(C) equal across the temperature range of interest.As shown in FIG. 8, a series resistance r_(C) is added as the frequencystabilizer circuitry 20 to the LC oscillator tank 10 to cause theinductor L and capacitor C to both have finite quality factors. Assumingthat the series resistors r_(L) and r_(C) are both temperaturedependent, the oscillation frequency using a phase condition ofφ_(Tank)=0 results in:

$\begin{matrix}{{\omega_{osc} = {\omega_{o} \cdot \sqrt{\frac{1 - {1/Q_{L}^{2}}}{1 - {1/Q_{C}^{2}}}}}},{{where}\text{:}}} & \left( {{Equation}\mspace{14mu} 9} \right) \\{{Q_{L} = \frac{\omega_{osc}L}{r_{L}}},{Q_{C} = {\frac{1}{\omega_{osc}{Cr}_{C}}.}}} & \left( {{Equation}\mspace{14mu} 10} \right)\end{matrix}$

In the case where C and L are temperature independent, examining theabove equations reveals that if Q_(L) and Q_(C) are equal across thetemperature range of interest, then the oscillation frequency, at zerophase, will simply be the natural frequency of the LC oscillator tank 10and independent of temperature. This is equivalent to moving thetemperature null (GNull) to the zero phase. In order to achieve this,r_(C) is selected to satisfy Q_(L)≈Q_(C). If Q_(L)=Q_(C), a temperaturenull at zero phase is attained with r_(L)=r_(c). If C and/or L aretemperature dependent, then a value for r_(C) is selected to result in aGNull around zero phase.

FIG. 9 is a flow diagram illustrating a method 200 for operating anLC-based oscillator at a non-zero phase in accordance with embodimentsof the present invention. In FIG. 9, once the temperature null phase ofthe LC oscillator tank is estimated at block 210, frequency stabilizercircuitry is provided within the LC oscillator that introduces in theoscillator open loop transfer function a phase opposite to the estimatedtemperature null phase of the LC oscillator tank at block 220. Then, atblock 230, the LC-based oscillator is operated such that the LCoscillator tank oscillates at the temperature null phase withoutviolating the Barkhausen phase criterion of zero open loop phase. Forexample, an amplifier within the LC-based oscillator can be designed toprovide, at steady state, a phase equal to −φ_(GNull) (negative of theGlobal Null Phase) for a zero open loop phase of the oscillator.

FIG. 10 is a diagram illustrating an exemplary LC-based oscillator 5designed to operate the LC oscillator tank 10 at the temperature nullphase in accordance with embodiments of the present invention. Asmentioned above, operating at the temperature null phase results in avery high stability oscillator 5 across temperature. In order toimplement an oscillator 5 that operates at the temperature null, afrequency stabilizer circuitry 20 is provided that enables the LCoscillator 5 to oscillate at zero open loop phase, while the LCoscillator tank 10 oscillates at the temperature null phase.

The frequency stabilizer circuitry 20 includes an amplifier (gmgtransconductor) 30 and phase shift circuitry (“e^(jφ)” block) 40. Theamplifier 30, in combination with the phase shift circuitry 40, isdesigned at steady state to provide a phase equal to the negative ofGNull (−φ_(GNull)). In addition, the amplifier 30 and phase shiftcircuitry 40 are designed such that the phase generated has a lowtemperature dependence. In an exemplary operation, the phase shiftcircuitry 40 shifts the phase of the output of the amplifier 30, whichcauses the LC oscillator tank 10 to oscillate with a non-zero phase ator near the tank's temperature null phase. For example, if the amplifierhas sufficient gain, the LC oscillator tank 10 will oscillate at afrequency where the phase across the LC oscillator tank 10 is equal andopposite to the phase produced by the phase shift circuitry 40.

The phase shift circuitry 40 allows the precise generation of such aphase and the ability to control the phase magnitude in order to be asclose as possible to the optimum phase operating point, via phasecontrol signal φ_(Control). Phase control signal φ_(Control) can be usedto set a fixed phase operating point, during calibration/trimmingoperations to tune the phase operating point and/or during real-timeoperations (i.e., while a device incorporating the oscillator 5 is inuse by a user) to fine tune the phase operating point. For example, acontroller/processor can measure the output of the oscillator 5 andgenerate the phase control signal φ_(Control) in response thereto. Theaccuracy of the phase control will determine how close the final phaseoperating point will be to the Global Null.

Thus, as shown in FIG. 10, the active circuitry (gm transconductor 30and phase shift circuitry 40) provides an equal and opposite phase suchthat the tank impedance at oscillation frequency is:

$\begin{matrix}{Z_{Tank} = {\frac{1}{g_{m}}^{- {j\phi}}}} & \left( {{Equation}\mspace{14mu} 11} \right)\end{matrix}$

Therefore, if the phase shift φ is close to the tank's φ_(Null),operation with an almost temperature independent frequency is achieved.The phase shift circuitry 40 can be implemented using active and/orpassive phase shifters with the ability to control the phase in order totune the oscillator to work as close as possible to the temperaturenull.

FIG. 11 is a diagram illustrating another exemplary oscillator 5designed to operate the LC oscillator tank at the temperature null phasein accordance with embodiments of the present invention. In the designof FIG. 11, two unequal currents (with ratio=m) having some phase shiftθ are injected into the tank 10 to produce an overall current I_(total)with a phase shift depending on the two components' ratio (m). Thecombined phase shift φ of I_(total) causes the LC oscillator tank 10 tooscillate at a non-zero phase. The ratio (m) and phase shift θ areselected to cause the LC oscillator tank 10 to oscillate with at or nearthe tank's temperature null phase.

For example, as shown in FIG. 11, a first amplifier (gm transconductor)30 a injects a current I1 into the tank 10, while a second amplifier(m*gm transconductor) 30 b, forming the frequency stabilizer circuitry20, injects a second current m*I1∠θ into the tank 10. The resultingphase produced in the LC oscillator tank 10 is a function of the ratio(m) of I1 and I2 and the phase of I2 (θ). For example, if m=1 and θ=90degrees, the resulting phase of the current through the LC oscillatortank would be −45 degrees. Therefore, to produce a phase shift ofmagnitude less than 45 degrees, the ratio (m) is typically less than 1and the phase shift θ may be less than 90 degrees.

The voltage (V∠θ) coupled to amplifier 30 b can be generated using anytechnique that is weakly temperature dependent. For example, multiplecoupled oscillators, i.e., “N” identical oscillators, designed togenerate multiple phases can produce the desired voltage (V∠θ). As anexample, the use of quadrature phase (θ=90°) where only two oscillatorsare needed can be used to produce the desired voltage.

One embodiment using quadrature phase is shown in FIG. 12. In FIG. 12,the oscillator 5 includes two nearly identical oscillator tanks 10 a and10 b oscillating in quadrature with respect to one another. Amplifiers30 b and 30 c produce nearly the same transconductance (g_(m)), whileamplifiers 30 a and 30 d produce respective phase shiftedtransconductances (m*g_(m)) to the tanks 10 a and 10 b. A gain block 60produces a phase shift of 180 degrees to force the two tanks 10 a and 10b to be 90 degrees out of phase with one another. Therefore, the totalphase shift φ across each tank 10 a and 10 b is simply a function of thecoupling ratio m and is given by:

tan(φ)=m  (Equation 12)

An automatic amplitude control (AAC) block 50 senses the operatingamplitude of the outputs V and V∠90 of the oscillator 5, andcontinuously adjusts the loop gain to be exactly unity, while theamplifiers 30 a-30 d are operating in their linear regime, as will bedescribed in more detail below in connection with FIG. 13.

Varying the ratio m may be done by either scaling the biasing current ofthe coupling amplifiers 30 a and 30 b and/or scaling the active devices(for example MOS transistor differential pair) dimensions within theamplifiers 30 a and 30 d. For example, varying the ratio m may be doneby switching off/on devices within amplifiers 30 a and 30 d, whilemaintaining low performance mismatch and a stable temperatureindependent coupling ratio across temperature, which in turn produces aconstant phase across the two tanks 10 a and 10 b. The setting of theratio m may be done during the trimming of the oscillator and can thenbe kept fixed during the operation of the oscillator. This may beachieved using analog or digital techniques. For example, a digital wordrepresenting the required ratio setting m may be stored on chip using anon-volatile memory to be available for future use during the lifetimeof the oscillator. It should be noted that the AAC block 50 shouldcontrol all transconductance amplifiers 30 a-30 d to ensure that allfour amplifiers have tracking gain, and therefore, the effectivecoupling ratio m remains constant during operation.

The two main sources of temperature dependence that affect the null are:(1) parasitic impedances of active circuitry that are temperaturedependent; and (2) harmonic content of the currents in the tank 10. Theparasitic capacitance, linear or nonlinear, of active circuitry maycause frequency shifts with temperature, supply and output voltageswing. Furthermore, the amplifier 30 output resistance may reduce thetank effective quality factor, thus reducing the tank's FOM. The effectof parasitics can be minimized by careful design of the oscillator 5.For example, the tank 10 capacitance can be designed to be much largerthan active circuitry parasitic capacitances to ensure that thetemperature dependence of the tank capacitance is dominant and anyparasitic capacitances have a very small impact on the temperature nullposition and quality. In addition, the output resistance can be designedto be as high as possible in order to avoid decreasing the qualityfactor of the tank 10, which in turn can degrade the quality of the nullas well as changing its position.

Since any oscillator by definition has a positive feedback loop, theoscillation amplitude in a classical oscillator with sufficient gainwill increase until the amplifier nonlinearities limit the amplitudegrowth. At steady state, the oscillator output contains many harmonicssuch that the large signal gain is unity, thus the amplitude isconstant. However, any nonlinearity in the active circuitry will injectharmonics into the tank 10, causing an energy imbalance between thecapacitor and the inductor, thus lowering the oscillation frequency torestore balance as given by:

$\begin{matrix}{\omega = {\omega_{os}\left( {1 - {\frac{1}{2Q^{2}}{\sum\limits_{2}^{\infty}{\frac{n^{2}}{n^{2} - 1}\frac{I_{n}^{2}}{I_{1}^{2}}}}}} \right)}} & \left( {{Equation}\mspace{14mu} 13} \right)\end{matrix}$

where ω_(os) is the oscillation frequency without harmonics and I_(n) isthe n^(th) harmonic of the current in the tank. As the harmonic contentinjected into the tank is temperature, supply and oscillation amplitudedependent, the oscillation frequency will also vary. The effect ofharmonics can be minimized by making the oscillator work at the criticalgain condition of oscillation, thus preventing any excess gain fromgrowing the oscillation amplitude to levels exceeding the amplifierlinear region, thus an oscillation with a very low harmonic content.

Turning now to FIG. 13, there is illustrated an exemplary AAC block 50designed to continuously adjust the loop gain to be exactly unity, whilethe amplifier 30 is operating in its linear regime. The AAC block 50includes amplitude sensing circuitry 52 that measures the amplitude ofoscillation, comparison circuitry 54 that compares the sensed amplitudeto a reference voltage Vref and a controller 56 that generates a signalto control the gain of the amplifier 30 based on the output of thecomparison circuitry 54. At steady state, the oscillation amplitudeshould be less than the linear region limit of the amplifier 30 andequal to a scaled value of the reference voltage, while the gain of theamplifier 30 is set to operate at the critical gain condition ofoscillation.

FIG. 14 illustrates an oscillator system 15 designed to operate the LCoscillator tank at the temperature null phase in accordance withembodiments of the present invention. The oscillator system 15 in FIG.14 includes a Phase-Locked Loop (PLL) or a Delay-Locked Loop (DLL) 60with a phase detector 62 that generates zero phase error output when itsinputs are shifted by a constant phase θ. For example, the phasedetector 62 can be an XOR gate or a mixer (for an analogimplementation), which at steady state will have its inputs inquadrature i.e. θ=90°.

The PLL/DLL 60 further includes a loop filter 64 and either a PLLvoltage controlled oscillator (VCO) 66 (when operating as a PLL) or aVoltage Controlled Delay Line (VCDL) 68 (when operating as a DLL). Whenoperating as a PLL, the output of the reference oscillator 5 (V) isinput to the phase detector 62 to enable the PLL 60 to lock the phase ofthe PLL VCO 66 to the reference oscillator 5, such that at steady state,the PLL VCO 66 and reference oscillator 5 have the same frequency andare shifted in phase by θ. Thus, the PLL 60 forces the PLL VCO 66 totrack the frequency of the reference oscillator 5, and therefore, thetwo oscillators 5 and 66 always have the same frequency even if they donot have the same tank. When operating as a DLL, the phase relationshipbetween the oscillator 5 and the DLL 60 is achieved using the VCDL 66.

The output V∠θ of the PLL VCO 66 or VCDL 68 is input as a feedbacksignal to the phase detector 62 and is also input to a programmablecoupling circuit 70. The programmable coupling circuit 70 couples afraction of the PLL/DLL output V∠θ, shown as m*I1∠θ, to the referenceoscillator 5 to enable the reference oscillator 5 to operate at thetemperature null. An AAC block 50 is also coupled to the oscillators 5and 66 in order to maintain the coupling ratio m between the twooscillators 5 and 66 constant over temperature. As such, the referenceoscillator 5 may operate at the required temperature null phase and havea low temperature dependence.

FIG. 15A illustrates a feedback aided oscillator 5 designed to operatethe LC oscillator tank 10 at the temperature null phase in accordancewith embodiments of the present invention. An oscillator system 15,shown in FIG. 15A, tunes the LC-tank 10, as shown in FIG. 8, and/or theactive circuitry (i.e., amplifier 30 in combination with phase shiftcircuitry 40), as shown in FIG. 10, of the oscillator 5 to adjust thetank phase shift to the temperature null phase.

The oscillator system 15 automates the operation at the GNull using afeedback control loop that stabilizes when the oscillator is in aspecific state of sensitivity to temperature. A temperature excitationsignal superimposed on the operating temperature is applied to theoscillator 5. For example, the temperature excitation signal may besinusoidal with a frequency f_(m) having an amplitude T_(m) andsuperimposed over an operating temperature T_(op). If the oscillator 5is sensitive to temperature, the oscillation 25 output from theoscillator 5 will change as the temperature associated with thetemperature excitation signal changes, resulting in an oscillation 25that is frequency modulated by f_(m).

The output 25 of the oscillator 5 is input to an FM demodulator 75,which senses the frequency of oscillation deviation. A Frequency ErrorGenerator (FEG) 80 converts the output (Freq Deviation) 78 of the FMdemodulator 75 and the temperature variation into an error signalFdT_(err) proportional to the sensitivity of the oscillation frequencyto temperature (Δf_(osc)/ΔT). The error signal (FdT_(err)) and areference signal (FdT_(ref)) are compared at comparison block 85 and theresultant is integrated at integrator 90 and then low pass filtered bylow pass filter 95. The final control signal (V_(ctrl)) 98 is then usedto control the oscillator 5 operation to allow the LC-tank 10 to operateat the temperature null phase or to shift the null to zero phase throughdirect control of the phase shift or by controlling r_(C) respectively.

An example of the implementation of the FEG 80 is shown in FIG. 15B. TheFEG 80 includes a mixer 82, a gain stage 84 and a transconductor 86. Themixer 82 takes as input the output of the FM Demodulator (FreqDeviation) and the temperature excitation signal (Temp Deviation) andgenerate an error signal (FdT_(err)) with an average value proportionalto Δf_(osc)/ΔT. The generated error signal (FdT_(err)) has a positiveaverage value for positive frequency deviations with increasingtemperature and reverses sign with negative frequency deviations, as canbe seen in the frequency plots of FIG. 15B.

At steady state, the error signal Fet_(ter) has an average value equalto the reference FdT_(ref). If the reference signal is zero, then thefeedback loop reaches steady state when the error signal has a zeroaverage. This is equivalent to tuning the oscillator 5 to operate at theLNull of the operating temperature, since a zero error signal isequivalent to Δf_(osc)/ΔT=0. If the reference signal is designed to be afunction of temperature with a zero value at a reference temperatureT_(ry), as shown in FIG. 15C, the loop stabilizes when:

$\begin{matrix}{\frac{\Delta \; f_{osc}}{\Delta \; T} = {\beta \cdot {FdT}_{ref}}} & \left( {{Equation}\mspace{14mu} 14} \right)\end{matrix}$

Overall, the loop is designed such that the oscillator operates at theGNull that has an LNull at temperature T_(r). The constant β isdependent on the loop gain and the target oscillation frequencytolerance with temperature.

Turning now to FIGS. 15D and 15E, in order to modulate the temperatureof the oscillator, integrated voltage controlled heating elements 300may be used that are capable of locally heating the oscillator 5. Anexample of a heating element is shown in FIG. 15D. The heating element300 includes an NMOS transistor 310 and an integrated polysiliconresistor 320. The input control voltage may be a rail to rail signal atthe desired modulation frequency and the duty cycle of the input signalcan be varied to control the amplitude to temperature modulation Tm, asshown in FIG. 15E. It may also be possible to control the temperature byvarying the amplitude of the input square wave.

The heating elements 300 can be distributed on the chip so that almostall devices of the oscillator system have the same temperature, and thusthe overall oscillator temperature may be controlled. The heatingelements 300 can be placed and wired so that, as much as possible, allconnecting wires are perpendicular to field lines and away fromsensitive circuit nodes and signals to avoid undesired electricalcoupling.

Not all LC-tanks have a deterministic temperature null phase due tomanufacturing process variations of the oscillator components, resultingin a variation of the temperature null phase. Therefore, trimmingtechniques may be needed to operate the LC-Tank oscillator substantiallyclose to the temperature null phase. The lower the desired oscillationfrequency variation with temperature (better frequency stability), thecloser the phase operation of the LC oscillator tank to the temperaturenull phase should be, and in turn, the more expensive the trimming maybe. In order to decrease the cost of trimming, the number and complexityof measurements should be minimized, as well as the number oftemperatures the measurements are performed at. The ability to trim theoscillator at room temperature only using few measurements or to designthe LC-tank oscillator to require minimum trimming may be of significanteconomical value.

Referring now to FIG. 16A, a differential tank 10 that uses anintentionally mismatched pair of tanks 10 a and 10 b in a differentialoscillator is shown. For example, values of L1/C1 and L2/C2 can bechosen such that the resonance frequency of L1/C1 in tank 10 b isslightly different than the resonance frequency of L2/C2 in tank 10 a.Such a tank 10 design showed an extended temperature null over a rangeof phases rather than a single phase, as shown in FIG. 16B. Thedifferential tank 10 design can be used in any of the above mentionedoscillator designs, as shown in FIGS. 8 and 10-15, to operate within thephase range that will result in low temperature dependence coveringmanufacturing process variations.

As mentioned above, using any of the oscillator designs shown in FIGS. 8and 10-15, once the oscillator is operating at the tank's temperaturenull, almost temperature independent oscillation frequency can beachieved. However, the null position may shift due to process variation.If this shift causes an acceptable temperature variation in frequency,depending on the application, no temperature trimming or calibrationwill be necessary. In this case, only a room temperature calibration forthe adjustment of initial frequency accuracy will be needed.

However, for more demanding applications, where the temperaturedependence of oscillation frequency should be minimized, temperaturetrimming to tune the oscillator to operate at the GNull once again maybe needed to assure operation at the tank's temperature null. Thetrimming procedure length and complexity determines the testing cost ofthe product, and in turn the overall cost of the final product.

The ability to relate the GNull to an LNull opens the way for low costtrimming, since the oscillator need not be operated at several or manytemperatures, which is expensive, to find the GNull. For example, asillustrated in FIGS. 17A and 17B, using two temperature insertions, ateach temperature, the phase shift across the tank is varied and theoscillation frequency is observed. Thus, φ_(Null) will be the phase withminimum frequency difference between two temperatures. Similarly, morethan two temperatures or insertions may be used to achieve the requiredtemperature dependence.

Referring now to FIGS. 18A and 18B, any oscillator, including but notlimited to LC-Tank based oscillators, crystal-based oscillators,MEMS-based oscillators, ring oscillators and relaxation oscillators,with sensitivity to temperature may be looked at as an oscillator with atemperature control, and thus the output frequency may vary in responseto a stimulus to the oscillator's temperature. If the temperature of theoscillator is modulated by a sinusoidal signal with frequency fm andamplitude Tm, as described above in connection with FIG. 15A, then usingthe Narrow Band Frequency Modulation (NBFM) approximation, it becomesapparent that the spectrum of the oscillator has a side tone at anoffset fm from the original oscillator frequency, as shown in FIG. 18B.The magnitude of this side tone is dependent on the modulation amplitudeTm, modulation frequency fm and the oscillator sensitivity totemperature BT. The magnitude of the side tone relative to theoscillation frequency magnitude Sr may be derived using the NBFMapproximation as:

Sr=Tm*BT/2*fm  (Equation 15)

From the above analysis, the oscillation frequency sensitivity totemperature BT of any oscillator, as defined above, may be estimated byobserving the spectral content of the oscillator output. The knowledgeof BT variation in response to oscillator parameter(s) at one or moretemperatures may be used to select such parameter(s) to operate theoscillator to exhibit certain desirable performance across temperature.The analysis assumes a sinusoidal temperature modulation for ease ofanalysis and simplicity. However, the temperature modulation profile canassume any shape which may be decomposed or approximated by discrete ora continuum of sinusoids thus make use of the above analysis inestimating the oscillator frequency temperature sensitivity BT.

Therefore, the oscillator can be trimmed, using, for example, thefeedback aided design shown in FIG. 15A, to operate at a specifictemperature sensitivity, including zero or very close to zerosensitivity at a single temperature To (e.g. room temperature), bymodulating the temperature by Tm around To with a frequency fm andmeasuring Sr. To find the minimum temperature sensitivity of theoscillator, the trimming control can be varied to search for the minimumSr that would be equivalent to operating the oscillator at minimumtemperature sensitivity. For example, the feedback loop shown in FIG.15A can be used to find the GNull simply by finding the LNull at roomtemperature, and the values obtained by the loop can be stored in anon-volatile memory for subsequent use.

FIG. 19 is a flow diagram illustrating a method 400 for determining thefrequency sensitivity of an oscillator to temperature in accordance withembodiments of the present invention. The method begins at block 410,where a temperature excitation signal is superimposed on an operatingtemperature of the oscillator. At block 420, an output of the oscillatoris demodulated to produce a frequency deviation signal indicative of afrequency deviation produced by the oscillator in response to thetemperature excitation signal. Then, at block 430, the frequencysensitivity of the oscillator to temperature is determined. For example,an error signal proportional to the frequency sensitivity of theoscillator to temperature can be produced from the frequency deviationsignal and the temperature excitation signal.

Oscillators designed to operate at the temperature null phase inaccordance with any of the above embodiments find applications in manydomains, such as but not limited to, consumer electronics, industrialequipment, computing devices, system interfaces (e.g., Universal SerialBus (USB), Serial Advance Technology Attachment (SATA) and PeripheralComponent Interconnect (PCI) Express), automotive, medical equipment,avionics, communications systems and various others. In addition,oscillators designed to operate at the temperature null phase inaccordance with any of the above embodiments can be standalonelow-temperature sensitivity oscillators, drop-in oscillators insystem-on-chips (SoC) and/or included within FPGA chips that are userprogrammable.

As may be used herein, the terms “substantially” and “approximately”provides an industry-accepted tolerance for its corresponding termand/or relativity between items. Such an industry-accepted toleranceranges from less than one percent to fifty percent and corresponds to,but is not limited to, component values, integrated circuit processvariations, temperature variations, rise and fall times, and/or thermalnoise. Such relativity between items ranges from a difference of a fewpercent to magnitude differences. As may also be used herein, theterm(s) “coupled to” and/or “coupling” and/or includes direct couplingbetween items and/or indirect coupling between items via an interveningitem (e.g., an item includes, but is not limited to, a component, anelement, a circuit, and/or a module) where, for indirect coupling, theintervening item does not modify the information of a signal but mayadjust its current level, voltage level, phase, and/or power level. Asmay further be used herein, inferred coupling (i.e., where one elementis coupled to another element by inference) includes direct and indirectcoupling between two items in the same manner as “coupled to”. As mayeven further be used herein, the term “operable to” indicates that anitem includes one or more of power connections, input(s), output(s),etc., to perform one or more its corresponding functions and may furtherinclude inferred coupling to one or more other items. As may stillfurther be used herein, the term “associated with”, includes directand/or indirect coupling of separate items and/or one item beingembedded within another item.

As will be recognized by those skilled in the art, the innovativeconcepts described in the present application can be modified and variedover a wide range of applications. Accordingly, the scope of patentssubject matter should not be limited to any of the specific exemplaryteachings discussed, but is instead defined by the following claims.

1. A low temperature sensitivity oscillator system, comprising: anLC-tank operating at a predetermined temperature null phase at which theoscillation generated by said LC-tank has a minimum frequency variationwith temperature changes across a specific temperature range; afrequency stabilizer circuitry comprising: an amplifier coupled in afeedback loop with the LC-tank; a phase shift circuitry coupled to theLC-tank and the amplifier to produce a phase substantially equal to thenegative of said temperature null phase based on a predetermined phasecontrol signal φ_(control), said phase produced by said phase shiftcircuitry being combined with the phase at which the LC-tank is operatedto sustain oscillation at the temperature null phase.
 2. Oscillatorsystem according to claim 1, wherein the phase control signalφ_(control) is substantially temperature independent and is fixed duringoperation of the oscillator system and stored on-chip.
 3. Oscillatorsystem according to claim 1, wherein it further comprises a resistancer_(c) in series with the capacitive part of the LC-tank to modify theposition of the temperature null phase of said LC-tank.
 4. Oscillatorsystem according to claim 3, wherein said the inductive part of theLC-tank is defined by an inductance L and a resistance r_(L), andwherein said resistance r_(c) is mounted in series with the capacitivepart is equal to the resistance r_(L) of the inductive part to set thequality factor Q_(L) of the inductive part equal to the quality factorQ_(c) of the capacitive part, the temperature null phase being equal tozero.
 5. Oscillator system according to claim 1, wherein it furthercomprises a capacitance having at least a second order temperaturedependence to decrease a frequency deviation of the LC-tank across atemperature range of interest, the capacitance including at least one ofan active capacitance and a passive capacitance.
 6. Oscillator systemaccording to claim 1, wherein the LC-tank includes a differential tankcomprising a mismatched pair of LC-tanks to produce a range of phases atwhich the temperature null phase can be set.
 7. Oscillator systemaccording to claim 1, wherein the frequency stabilizer circuitry furthercomprises an automatic amplitude control circuitry including anamplitude sensing circuitry to measure the amplitude of the oscillationgenerated by said oscillator system, and a controller to control thegain of the amplifier to prevent the amplitude of said oscillation fromexceeding a predefined limit.
 8. Oscillator system according to claim 1,wherein the frequency stabilizer circuitry further includes: a firstamplifier suitable for injecting a first current into the LC-tank; asecond amplifier suitable for injecting a second current into theLC-tank; said second current being phase shifted relative to the firstcurrent to cause the LC-tank to operate at the temperature null phase.9. Oscillator system according to claim 8, wherein it comprises: a firstLC-tank coupled with a first and a second amplifiers a second LC-tankcoupled with a third and a fourth amplifiers said first and secondLC-tanks oscillating in quadrature with respect to one another; saidsecond and third amplifiers producing currents of the same magnitudeg_(m)*V; said first and fourth amplifiers producing a phase-shiftedcurrent of magnitude m*g_(m)*y relative to second and third amplifiercurrents respectively; such that each said first and second LC-tanksoperates at their temperature null phase.
 10. Oscillator systemaccording to claim 9, wherein it further comprises an automaticamplitude control circuitry coupled to first and second LC-tanks tosense the amplitude of the respective output oscillations of said firstand second LC-tank to prevent the amplitude of said oscillations fromexceeding a predefined limit.
 11. Oscillator system according to claim8, wherein the LC-tank and the amplifier comprise a reference oscillatorand wherein the frequency stabilizer circuitry further includes: a phaselocked loop or PLL having: a phase detector coupled to receive theoutput oscillation and a feedback signal and operable to generate aphase error output signal indicative of a difference in phase betweenthe output oscillation and the feedback signal, the phase error outputsignal being equal to zero when the output oscillation and the feedbacksignal are shifted by a constant phase; and a PLL oscillator coupled toreceive the phase error output and operable to produce a PLL oscillationas the feedback signal; and a programmable coupling circuit coupled toPLL oscillator and to the reference oscillator and operable to provide afraction of the PLL oscillation to the reference oscillator to cause theLC oscillator tank of the reference oscillator to oscillate at thetemperature null phase; and automatic amplitude control circuitrycoupled to reference oscillator and PLL oscillator to sense theamplitude of the respective output oscillations to prevent the amplitudeof said oscillations from exceeding their respective predefined limits.12. Oscillator system according to claim 8, wherein the LC-tank and theamplifier comprise a reference oscillator and wherein the frequencystabilizer circuitry further includes: a delay locked loop or DLLhaving: a phase detector coupled to receive the output oscillation and afeedback signal and operable to generate a phase error output signalindicative of a difference in phase between the output oscillation andthe feedback signal, the phase error output signal being equal to zerowhen the output oscillation and the feedback signal are shifted by aconstant phase; and a voltage controlled delay line coupled to receivethe phase error output and operable to produce the feedback signal; anda programmable coupling circuit coupled to voltage controlled delay lineand to the reference oscillator and operable to provide a fraction ofthe feedback signal to the reference oscillator to cause the LCoscillator tank of the reference oscillator to oscillate at thetemperature null phase; and automatic amplitude control circuitrycoupled to reference oscillator to sense the amplitude of the outputoscillations to prevent the amplitude of said oscillations fromexceeding their predefined limit.
 13. Oscillator system according toclaim 1, wherein the LC-tank and the amplifier comprise an oscillator,and wherein the frequency stabilizer circuitry further comprises: afrequency stabilizer feedback loop including a heating element operableto apply a temperature excitation signal superimposed on an operatingtemperature of the oscillator; a demodulator coupled to the oscillatorto receive the output oscillation and operable to demodulate the outputoscillation to produce a frequency deviation signal indicative of afrequency deviation produced by the oscillator in response to thetemperature excitation signal; a frequency error generator coupled toreceive the frequency deviation signal and the temperature excitationsignal and operable to produce an error signal proportional to thesensitivity of the oscillator to temperature; and a comparator coupledto receive the error signal and a reference signal and operable toproduce a control signal to control the temperature dependence of theoscillator which is used to operate the LC oscillator tank at thetemperature null phase; and wherein, at steady state, the error signalhas an average value equal to the reference signal.
 14. Oscillatorsystem according to claim 13, wherein the frequency error generatorincludes a mixer coupled to receive the frequency deviation signal andthe temperature excitation signal and operable to generate the errorsignal with an average value proportional to the frequency deviationwith temperature, wherein the generated error signal has a positiveaverage value for positive frequency deviations with increasingtemperature and a negative average value for negative frequencydeviations with increasing temperature.
 15. Oscillator system accordingto claim 13, wherein the frequency stabilizer feedback loop furtherincludes: an integrator coupled to the comparator to integrate an outputof the comparator and produce an integrated signal; and a low passfilter coupled to receive the integrated signal and operable to filterthe integrated signal to produce the control signal.
 16. Oscillatorsystem according to claim 13, wherein the frequency stabilizer feedbackloop uses a temperature dependent reference signal with value zero at aparticular reference temperature such that the oscillator system has anoscillation frequency of low temperature dependence across a temperaturerange of interest, said oscillator system operating at a localtemperature null at said reference temperature and at a globaltemperature null across the temperature range of interest. 17.Oscillator system according to claim 13, wherein the frequencystabilizer feedback loop operates to trim the oscillator control signalto operate the oscillator at a minimum oscillation frequency temperaturesensitivity at a particular temperature by setting the reference signalto a value tending to zero; and wherein the steady state value of theoscillator control signal is stored on-chip to be used by oscillator tohave minimal temperature sensitivity across a temperature range aroundthe reference temperature used during trimming.
 18. Oscillator systemaccording to claim 1, wherein the frequency stabilizer circuitry isoperable to measure a frequency of the output oscillation across variousphases of the LC-tank by sweeping the oscillator phase control signalacross at least two temperatures to determine the temperature null phaseas the phase with a minimum frequency difference between the at leasttwo temperatures.
 19. A method for determining the frequency sensitivityof an oscillator to temperature, comprising: applying a temperatureexcitation signal superimposed on an operating temperature of theoscillator; demodulating an output of the oscillator to produce afrequency deviation signal indicative of a frequency deviation producedby the oscillator in response to the temperature excitation signal; andproducing an error signal from the frequency deviation signal and thetemperature excitation signal proportional to the frequency sensitivityof the oscillator to temperature.
 20. A temperature excitation system togenerate a temperature excitation signal superimposed on an operatingtemperature of an oscillator, wherein it comprises: distributedintegrated heater elements placed and wired such that all connectingwires are perpendicular to field oscillator lines and away fromsensitive circuit nodes and signals to avoid undesired electricalcoupling; and a temperature modulation profile of a shape which isdecomposed or approximated by discrete or a continuum of sinusoidsincluding a square wave with varying amplitude and duty cycle to controlthe heating intensity and its variation with time.